Micro-crystalline material and method of preparation

ABSTRACT

The invention relates to a method of making a refractory microcrystalline material by devitrification of a melt of a base glass composition containing Al2O3, SiO2 and, optionally, CaO and MgO, and as additives to the base glass composition, iron oxide and chromium oxide, wherein devitrification is carried out by cooling the melt without re-heating the melt at any stage; and to a refractory micro-crystalline material.

United States Patent Rogers et al.

MICRO-CRYSTALLINE MATERIAL AND METHOD OF PREPARATION Inventors: Philip Sydney Rogers; James Williamson, both of London; Peter Edwin Johnson, East Barnet, all of England Assignee: National Research Development Corporation, London, England Filed: Jan. 22, 1974 App]. No.: 435,508

Foreign Application Priority Data Feb. 2, 1973 United Kingdom 5408/73 US. Cl 106/39.6; 65/33 Int. Cl C03b 29/00 Field of Search 65/33; 106/396 References Cited UNITED STATES PATENTS 6/1915 Brookfielcl 65/33 [451 Aug. 26, 1975 1,893,382 11/1933 Watson 65/33 3,205,079 9/1965 Stookey 65/33 X 3,268,315 8/1966 Stookey 65/33 3,557,575 1/1971 Beal 65/33 3,677,728 7/1972 Kitaigorodsky et a1..... 65/33 3,804,608 4/1974 Gaskell et a1. 65/33 3,819,387 6/1974 Leger et a1.... 65/33 X Primary ExaminerS. Leon Bashore Assistant ExaminerFrank W. Miga Attorney, Agent, or Firm-Watson, Cole, Grindle & Watson 5 7] ABSTRACT 15 Claims, N0 Drawings MICRO-CRYSTALLINE MATERIAL AND METHOD OF PREPARATION BACKGROUND OF THE INVENTION bined with hardness, impermeability and resistance to chemical attack and abrasion. Certain of the glass forming techniques which are used in the production of glass-ceramics are rendered more favourable by adoption of a heat treatment which does not require a re heating stage e.g. rolling, casing and pressing. Hence, products made of the glass-ceramics of the invention beneficially take advantage of the favourable physical properties of the material and the convenience of the forming and shaping techniques. Examples of the application of these materials are as follows:

a. Pipelines, nozzles, unions and valve seats, for conveying corrosive fluids and fluids carrying abrasive solid particles, e.g. drain pipes and guttering.

b. Wear resistant ducts and sheets, for chutes and paving.

c. Kerbstones and flagstones.

d. Flat (or corrugated) sheet, for certain walling cladding or tiles.

e. Prefabricated linings for tunnels.

f. Bottles and containers.

g. Low grade refractories e.g. furnace linings and supports for electrical heating elements.

h. Fan blades.

In the past, micro-crystalline glass-ceramics have been produced by cooling a melt of a glass composition which often contains a metallurgical slag, and a nucleating agent and thereafter re-heating the glass composition in order to make it crystalline. For example, United Kingdom Pat. No. l,l52,4l7 describes the preparation of micro-crystalline materials by devitrification of a glass made from a metallurgical slag, the material consisting of SiO MgO, A1 and Cr O in the relative proportions, by weight, of 46 to 62% SiO 21 to 34% MgO, 10 to 21% A1 0 and 1.5 to 5% Cr O and optionally containing CaO and FeO in the relative proportions, by weight, of up to 2% CaO and up to 1.5% F eO. Although it is stated in the specification that a limited range of the compositions will crystallise merely by cooling at a suitable rate, the specification does not teach which compositions fall within the limited range or the way in which the cooling is to be carried out.

It is an object of the present invention to produce a micro-crystalline material by the controlled cooling of a melt of a particular range of glass compositions without any need for re-heating.

SUMMARY OF THE INVENTION The invention provides a method of making a microcrystalline material comprising the steps of 1) preparing a melt of a composition comprising a base glass composition consisting of from 0 to 30 weight percent MgO, from 5 to 35 weight percent A1 0 from 35 to 75 weight percent SiO and from 0 to 30 weight percent CaO; from 0.5 to 3 parts by weight chromium oxide (expressed as Cr O per 100 parts by weight of the base glass composition and from 0.5 to parts by weight iron oxide (expressed as mo.) per 100 parts by weight of the base glass composition. and (2) Cooling the melt at such a rate that initially crystals of spine],

mostly less than lum in size, are formed within the melt and subsequently crystals of a silicate phase grow upon the spinel crystals to yield the micro-crystalline material.

The invention also provides a micro-crystalline material comprising from 0 to 30 weight percent MgO, from 5 to 35 weight percent A1 0 from 35 to weight percent SiO from greater than 8.5 to less than 30 weight percent CaO, from 0.5 to 3 parts by weight chromium oxide (expressed as Cr O per part by weight of the combined MgO, A1 0 SiO and CaO, and from 0.5 to 10 parts by weight iron oxide (expressed as Fe O per 100 parts by weight of the combined MgO, Al O SiO and CaO.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It may be advantageous to employ greater than 2, greater than 8.5 or in some cases greater than 12 weight percent CaO.

It may be advantageous to employ from 5 to 30 weight percent MgO.

Spinel describes members of a group of minerals having the general formula AO.B O where AO represents the oxide of a divalent metal and B 0 represents the oxide of a trivalent metal. A may, for example, be Mg or Fe. B may, for example, be Al, Cr or Fe. Extensive solid solution can occur between two or more end members of the spinel group and two or more spinel solid solutions can exist together. During the second stage of the cooling process crystals of a silicate phase grow upon the spinel crystals by a heterogeneous nucleation process. The silicate crystals so formed are usually of mineral phases belonging to the pyroxene group which describes members of a group of metasilicate minerals, particular examples being enstatite and clino-enstatite both having the composition MgO.SiO and diopside having the composition CaO.MgO.2SiO Extensive solid solution can take place between different members of the pyroxene group.

Once crystals of a silicate phase have begun to grow upon the spinel crystals, the remaining glass is then fully or partially divitrified. The final micro-cyrstalline material may still contain up to 20 weight percent re sidual glass. The proportion of residual glass can be controlled as required and for most applications a residual glass content of up to 5 weight percent is suitable.

Chromium oxide and iron oxide are added to the base glass composition to control the formation of heterogeneous nuclei i.e. spinel crystals. The chromium oxide may be added to the batch before or after melting either as an oxide of chromium, as a chromate or some other convenient compound of chromium. When ion blast-furnace slag is one of the constituents, chrome ore can conveniently be used as a source of chromium oxide and iron oxide. Similarly, the iron oxide may be added to the batch before or after melting either as an oxide of iron e.g. haematite or magnetite, a hydrated oxide of iron or some other convenient compound of iron such as a mineral containing iron oxide. The chromium and iron compounds may be added together or separately.

When iron oxide is used, the most convenient form is ferric oxide, F8203, since this most readily provides the trivalent cations required for the formation of spinel.

Further, when iron oxide is used, the compon .ts of the glass composition are preferably melted togetner in an oxidising atmosphere e.g. air, so that most of the iron is present in the glass in the ferric state. Under these conditions, iron oxide additive need only be present in from 0.5 to parts by weight per 100 parts of the base glass. 1f the melting furnace atmosphere is more reducing, it is thought that more iron oxide may be required from 0.5 to parts by weight per 100 parts of the base glass) since a higher proportion of the iron'will be in the ferrous (divalent) state.

It may be advantageous to 'add' further transition metal oxides. These include the oxides of zinc, manganese vanadium, molybdenum, cobalt and nickel. I

It may be advantageous to add Na O in an amount of up to 5 parts by weight per 100 parts of the base glass to the glass composition to assist in refining and homogenising the melt and to reduce the melting temperature. Other fluxing and refining agents, such as fluorides and phosphates, may also be beneficially employed.

The constituents of the base glass may be derived from metallurgical slag if so desired. A metallurgical slag would also contribute other constituents to the melt, for example, manganese oxide.

The cooling schedule employed to produce the devitrification depends on the composition of the melt. The cooling rate is adjusted so that initially large numbers of.spinel.crystals form in the melt and thereafter crystals of a silicate phase grow upon thespinel nuclei. Cooling may be continuous in a series of different rates or the temperature of the melt may be held constant at one or more stages during the cooling schedule. The critical range of temperature over which the cooling must be controlled is from the temperature of melting down to 800C, or to the chosen holding temperature, whichever is the higher. Preferably the holding temperature is maintained for a period of time of from 0 to 3 hours, more preferably to 1 /2 hours at between the temperature of melting and 800C. The holding temperature should not be maintained for so long a period that excessive recrystallisation takes place. To attain a microcrystalline material having a grain size less than 2am it is normally necessary to use a holding temperature in the range of from 800 to ll00C., preferably from 900 to 1000C. Compositions which give a micro-crystalline material when a holding temperature in the range 800 to ll00C. is used can be devitrified to give coarse-grained products by holding at a higher temperature, for example in the range of from 1 100 to 1 200C. should a grain size greater then 2am be required. The ultimate grain size of the glass-ceramic material is controlled by (a) the composition of the melt and (b) the cooling schedule employed.

The invention is further illustrated with reference to the following Examples:

EXAMPLE 1 Base glass: (a0 wt7: Additions to MgO 14 base glass parts (3 0;, 2 A1 0 10 per 100 parts Fla- 0;, 3 SiO, 56 of base glass) M1 0 3 One composition was melted at 1450K for 3 hours in air, placed in furnace at 950C. for 1 hour and thereafter cooled at natural rate to room temperature.

The other composition was melted at l450C. for 3 hours in air, placed in a furnace at 950C. for 3 hours and thereafter cooled at natural rate to room temperature. The modulus of rupture (0') was measured for six samples of the resulting material and the average value with mean deviation was calculated:

0' 182.2 i 14.8 MN/m (26,400 psi).

EXAMPLE 2 Two'compositions of the following constituents were prepared:

Basc glass:

blastfurnacc slag* 50wt% Additions to Cr O 2 MgO 10 Base glass Fe o 2 SiO 40 Na O 3 One composition was melted at l450C for 3 hours in air, placed in a furnace at 1000C. for 1 hour and thereafter cooled at natural rate to room temperature. The modulus of rupture was calculated from seven samples of the resulting material:

0" 141.2 1 9.7 MN/m (20,500 psi).

The other composition was melted at 1500C. for 3 hours in air, placed in a furnace at 1000C. for 1 hour, and cooled at natural rate to room temperature. The

modulus of rupture was calculated from six samples of the resulting material:

0' 145.9 1 23.7 MN/m (21,200 p.s.i)..

EXAMPLE 3 Two compositions of the following constituents were prepared:

Base glass:

blast-furnace slag* wt7r Additions to C 0 2 MgO 5 base glass Fe O 2 SiO: 20 Na O 1.5

One composition was melted at l450C. for 3 hours in air, placed in a furnace at 900C. for 1 hour and thereafter cooled at natural rate to room temperature.

The other composition was melted at l450C. for 3 hours, placed in a furnace at 900C. for 3 hours and cooled at natural rate to room temperature. The modulus of rupture was calculated from six samples of the resulting material:

0'= 144.4 i 12.3 MN/m (20,900 psi).

EXAMPLE 4 A composition of the following constituents was prepared:

Base glass: blast furnace slag* 75wt/r- Additions to C 0 2 SiO, 25 base glass Fe O 2 "Composition of slag used in Examples 2 to 4 (to nearest 0.5 \Vl'Yr) 0.10 39.5 MgO 7.0 A1 0 19.0 SiO 31.5 P00 1.0 MnO 1.0 S 1.5

The composition was melted at 1500C for 3 hours in air, placed in a furnace at 920C for 2 hours and thereafter cooled at natural rate to room temperature.

Crystal sizes in Examples 1 to 4 lay in the range to 2pm, but mostly in the range 0.5 to 15pm.

The modulus of rupture was measured by a three point loading technique, with the lower knife edges 2 cms apart, and a centrally placed upper knife edge. The specimens were about 30 mm. long, 2.5 mm. deep and mm. broad, and were prepared on a surface grinder using a wheel impregnated with 200 grit diamond.

1 claim:

1. A method of making a micro-crystalline material comprising the steps of l. preparing a melt of a composition comprising a base glass composition consisting of from 0 to 30 weight percent MgO, from 5 to 35 weight percent A1 0 from 35 to 75 weight percent SiO and from 0 to 30 weight percent CaO; from 0.5 to 3 parts by weight chromium oxide (expressed as Cr O per 100 parts by weight of the base glass composition and from 0.5 to parts by weight iron oxide (expressed as Fe O per 100 parts by weight of the base glass composition, and

. cooling the melt at such a rate that initially crystals of spinel, mostly less than 1pm in size, are formed within the melt and subsequently crystals of a sili cate phase grow upon the spinel crystals to yield the micro-crystalline material.

2. A method according to claim 1 wherein the base glass composition contains greater than 8.5 to less than 30 weight percent CaO.

3. A method according to claim 1 wherein the base glass composition contains from 5 to 30 weight percent MgO.

4. A method according to claim 1 wherein the composition contains from 0.5 to 5 parts by weight iron oxide (expressed as Fe O per 100 parts by weight of the base glass composition and the melt is prepared in an oxidising atmosphere.

5. A method according to claim 1 wherein on cooling the temperature of the composition is held constant for up to 3 hours at a temperature between the temperature of melting and 800C.

6. A method according to claim 5 wherein the temperature of the composition is held constant at a temperature of from 800 to 1 100C.

7. A method according to claim 1 wherein said melt is prepared in an oxidizing atmosphere and wherein said cooling step comprises first cooling the melt to a temperature of from 900 to 1000C and holding the composition at this temperature for from to 1 /2 hours, and subsequently cooling the composition to ambient temperature.

8. A composition usable for preparing a microcrystalline material in accordance with the method of claim 1 comprising from 0 to 30 weight percent MgO, from 5 to 35 weight percent A1 0 from 35 to weight percent SiO from O to 30 weight percent CaO, from 0.5 to 3 parts by weight chromium oxide (expressed as Cr O per parts by weight of the combined MgO, Al O SiO and CaO, and from 0.5 to 10 parts by weight iron oxide (expressed as Fe O per 100 parts by weight of the combined MgO, A1 0 SiO and CaO.

9. A composition according to claim 8 comprising from greater than 8.5 to less than 30 weight percent CaO.

10. A composition according to claim 8 comprising from 5 to 30 weight percent MgO.

11. A composition according to claim 8 comprising 0.5 to 5 parts by weight iron oxide (expressed as Fe o per 100 parts by Weight of combined MgO, A1 0 SiO and CaO.

12. A micro-crystalline material comprising crystals of spinel less than l,u,m in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim 1.

13. A microcrystalline material comprising crystals of spinel less than 11am in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim 2.

14. A miero-crystalline material comprising crystals of spinel less than 1am in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim 3.

15. A micro-crystalline material comprising crystals of spinel less than l,u.m in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim 4. 

1. A METHOD OF MAKING A MICRO-CRYSTALLINE MATERIAL COMPRISING THE STEPS OF
 1. PREPARING A MELT OF A COMPOSITION COMPRISING A BASE GLASS COMPOSITION CONSISTING OF FROM 0 TO 30 WEIGHT PERCENT MGO, FROM 5 TO 35 WEIGHT PERCENT AL2O3, FROM 35 TO 75 WEIGHT PERCENT SIO2, AND FROM 0 TO 30 WEIGHT PERCENT CAO, FROM 0.5 TO 3 PARTS BY WEIGHT CHROMIUM OXIDE (EXPRESSED AS CR2O3) PER 100 PARTS BY WEIGHT OF THE BASE GLASS COMPOSITION AND FROM 0.5 TO 10 PARTS BY WEIGHT IRON OXIDE (EXPRESSED AS FE2O3) PER 100 PARTS BY WEIGHT OF THE BASE GLASS COMPOSITION, AND
 2. COOLING THE MELT AT SUCH A RATE THAT INITIALLY CRYSTALS OF SPINEL, MOSTLY LESS THAN 1 UM IN SIZE, ARE FORMED WITHIN THE MELT AND SUBSEQUENTLY CRYSTALS OF A SILICATE PHASE GROW UPON THE SPINEL CRYSTALS TO YIELD THE MICRO-CRYSTALLINE MATERIAL.
 2. cooling the melt at such a rate that initially crystals of spinel, mostly less than 1 Mu m in size, are formed within the melt and subsequently crystals of a silicate phase grow upon the spinel crystals to yield the micro-crystalline material.
 2. A method according to claim 1 wherein the base glass composition contains greater than 8.5 to less than 30 weight percent CaO.
 3. A method according to claim 1 wherein the base glass composition contains from 5 to 30 weight percent MgO.
 4. A method according to claim 1 wherein the composition contains from 0.5 to 5 parts by weight iron oxide (expressed as Fe2O3) per 100 parts by weight of the base glass composition and the melt is prepared in an oxidising atmosphere.
 5. A method according to claim 1 wherein on cooling the temperature of the composition is held constant for up to 3 hours at a temperature between the temperature of melting and 800*C.
 6. A method according to claim 5 wherein the temperature of the composition is held constant at a temperature of from 800* to 1100*C.
 7. A method according to claim 1 wherein said melt is prepared in an oxidizing atmosphere and wherein said cooling step comprises first cooling the melt to a temperature of from 900* to 1000*C and holding the composition at this temperature for from 3/4 to 1 1/2 hours, and subsequently cooling the composition to ambient temperature.
 8. A COMPOSITION USABLE FOR PREPARING A MICROCRYSTALLINE MATERIAL IN ACCORDANCE WITH THE METHOD OF CLAIM 1 COMPRISING FROM 0 TO 30 WEIGHT PERCENT MGO, FROM 5 TO 35 WEIGHT PERCENT A12O3, FROM 35 TO 75 WEIGHT PERCENT SIO2, FROM 0 TO 30 WEIGHT PERCENT CAO, FROM O.5 TO 3 PARTS BY WEIGHT CHROMIUM OXIDE (EXPRESSES AS CR2O3) PER 100 PARTS BY WEIGHT OF THE COMBINED MGO, A12O3, SIO, AND FROM 0.5 TO 10 PARTS BY WEIGHT IRON OXIDE (EXPRESSED AS FE2O3) PER 100 PARTS BY WEIGHT OF THE COMBINED MGO, A12O3, SIO2 AND CAO.
 9. A composition according to claim 8 comprising from greater than 8.5 to less than 30 weight percent CaO.
 10. A composition according to claim 8 comprising from 5 to 30 weight percent MgO.
 11. A composition according to claim 8 comprising 0.5 to 5 parts by weight iron oxide (expressed as Fe2O3) per 100 parts by weight of combined MgO, Al2O3, SiO2 and CaO.
 12. A micro-crystalline material comprising crystals of spinel less than 1 Mu m in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim
 1. 13. A micro-crystalline material comprising crystals of spinel less than 1 Mu m in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim
 2. 14. A micro-crystalline material comprising crystals of spinel less than 1 Mu m in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim
 3. 15. A micro-crystalline material comprising crystals of spinel less than 1 Mu m in size and crystals of silicate phase grown on said spinel crystals, said material being made by the method of claim
 4. 